The present invention relates to a charging apparatus which employs a contact type charging method which utilizes electrically conductive particles. It also relates to a process cartridge.
In the past, a corona based charging apparatus (corona discharger) has been widely used as a charging apparatus for uniformly charging (or discharging) an image bearing member (object to be charged), such as electrophotographic photoconductive member or an electrostatically recordable dielectric member, in an image forming apparatus such as an electrophotographic apparatus or an electrostatic recording apparatus.
In recent years, a contact type charging apparatus (contact charging apparatus) which employs a contact charging method, has been put to practical use, because a contact charging apparatus is advantageous over a corona discharger in that the former produces a smaller amount of by-products such as ozone, and also has less power consumption, than the latter. According to this contact charging method, an object is charged by placing a charging member in contact with the object to be charged, while applying voltage to the charging member.
However, in principle, even a contact charging apparatus produces active ions such as ozone, although only an extremely small amount compared to a corona discharger. In other words, it is not completely free from the problems related to the production of an active ion such as ozone.
In order to solve the above described problems, an injection charge mechanism has been proposed, which will be described next.
The injection charge mechanism charges the surface of an object by injecting electric charge directly from a contact charging apparatus into the object. This mechanism has been disclosed in U.S. Pat. Nos. 6,134,407, 6,081,681, 6,128,456, and, the like.
In the injection charge mechanism, electric charge is directly injected into the peripheral surface of an object be charged, by placing a contact charging member, the electrical resistance of which is in the mid range, in contact with the peripheral surface of the object. Thus, in principle, electric discharge and the mechanism therefor are not involved in the charging of the object. Therefore, even when the value of the voltage applied to the contact charging member is no more than the threshold value, the object can be charged to a potential level equivalent to the value of the voltage applied to the contact charging member. FIG. 5 shows the comparison in charge characteristic between an injection-based charging mechanism (solid line B) and corona-based charging mechanism (broken line A), that is, the conventional charging mechanism. The charging mechanism based on direct injection is not accompanied by ion production. Therefore, it does not suffer from the problems caused by the byproducts of electric discharge.
More concretely, the direct injection charging mechanism is such a charging mechanism in which electric charge is directly injected into the traps in the peripheral surface of an object (image bearing member) to be charged, or into the constituents, such as electrically conductive particles, of the charge injection layer of the object (image bearing member), by applying voltage to a contact charging member such as a magnetic brush. In the direct injection charging mechanism, electric discharge is not the dominant factor. Therefore, the value of the voltage necessary for charging an object (image bearing member) has only to be equal to the value of the desired voltage level to which the peripheral surface of the image bearing member is to be charged, and in addition, it produces a very small amount of ozone.
In the case of a magnetic brush employed as a charging member, it is conceivable to make the particle size of the magnetic particles microscopic in order to improve the magnetic brush in terms of the uniformity with which an object is charged. However, if the particle size of the magnetic particles is simply reduced to the microscopic level, it becomes easier for the magnetic particles to fall off the charging device. If the microscopic magnetic particles fall off the charging device, they mix into the developing device, adversely affecting the performance of the developing device, and also, they appear, as parts of a completed image, on a recording medium through the transfer and fixing processes, degrading the image quality. In order to prevent the magnetic particles from falling off the charging device, it is possible to improve the magnet in terms of magnetic force, that is, the magnetic toner retaining force, and/or to improve the magnetic particles in terms of magnetic properties. However, both ideas lead to cost increases.
In other words, the magnetic brush is difficult to further improve in terms of charge uniformity.
Thus, a few proposals have been made as means for improving the magnetic brush, as a charging member, in terms of charge uniformity. According to one of them, a porous roller, for example, a sponge roller, as a contact member for directly injecting electric charge, is coated with electrically conductive microscopic particles (which hereinafter way be referred to simply as conductive particles) in order to improve the contact charging member. This structural arrangement provides virtually ideal contact (electrical contact), between a contact charging member and an object to be charged, enabling the contact charging member to satisfactorily charge the object.
The primary object of the present invention is to provide: a charging apparatus, which employs a charging method virtually free of byproducts such as ozone, and is superior in charge uniformity; a process cartridge, which employs such a charging apparatus; and an image forming apparatus, which employs such a charging apparatus or such a process cartridge.
Another object of the present invention is to provide: a charging apparatus, the performance of which in terms of charge uniformity remains stable for a long period of usage; a process cartridge, which employs such a charging apparatus; and an image forming apparatus, which employs such a charging apparatus or such a process cartridge.
Another object of the present invention is to provide: a charging apparatus, which is capable of providing an optimal state of contact between the charging device and object to be charged; a process cartridge employing such a charging apparatus; and an image forming apparatus employing such a charging apparatus or such a process cartridge.
These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings.